Droopy Cake- Need Help

Okay, I was making a gift bag cake. It was 4- 2" layers with icing in the middle of each layer. I covered the whole thing in mmf and decorated, by that night the whole back side was drooping. So much that it started tearing on the sides- so my question is how do you get it not to droop? Does anyone put dowel rods in it to hold it in place? What to do?

It looks like it was a support problem...did you put dowels in the first 2 layers before stacking the other 2 (on a cake board) on top? It could also be due to the cake not setteling before the fondant was applied.